Organic light-emitting devices using organic substances seem to have a bright prospect as solid, light-emitting inexpensive devices for large-area full color displays or as light source arrays for recording, and many developments have been made on them. The organic light-emitting devices are generally constituted by a light-emitting layer interleaved between a pair of opposed electrodes. Light emission is a phenomenon which occurs when electrons and holes are respectively injected from the cathode and the anode upon application of an electric field across the electrodes, and are recombined in the light-emitting layer and their energy level returns from conduction band to valence band with emitting the energy as light.
Conventional organic light-emitting devices have required a high driving voltage, with providing a low light-emitting luminance and a low light-emitting efficiency. In recent years, however, various techniques have been reported to solve the problems. For example, there is known an organic light-emitting device having an organic thin film formed by vapor deposition of an organic compound (Applied Physics Letters, vol. 51, p.913, 1987). The organic light-emitting device described there has a two-layered structure of an electron transporting layer comprising an electron transporting material and a hole transporting layer comprising a hole transporting material, and shows a markedly improved light-emitting performance in comparison with conventional single-layered devices.
The organic light-emitting device described above uses a low molecular amine compound as a hole transporting material and an Al complex of 8-quinolinol (Alq) as an electron transporting and light-emitting material, with the color of emitted light being green. After the report, a number of such vapor deposition type organic light-emitting devices have been reported (see references described in Macromolecular Symposium, vol. 125, p. 1, 1997).
However, the organic light-emitting devices involve a serious problem of an extremely low light-emitting efficiency in comparison with inorganic LED devices or fluorescent tubes. Most of the presently proposed organic light-emitting devices utilize fluorescent light emission obtained from singlet excitons of organic compound light-emitting materials. Based on the simple mechanism of quantum chemistry, the ratio of singlet excitons generating fluorescence to triplet excitons generating phosphorescence in number is 1:3. Hence, as long as fluorescence is utilized, only 25% of the generated excitons can be effectively utilized, thus light-emitting efficiency being unavoidably low. On the other hand, if phosphorescence obtained from the triplet excitons can be utilized, it serves to improve light-emitting efficiency. Under such idea, a phosphorescence-emitting device using an iridium-phenylpyridine complex has recently been reported (Applied Physics Letter, vol. 75, p. 4, 1999; Japanese Journal of Applied Physics, vol. 38, p.L1502, 1999). In these reports, the light-emitting devices are reported to show light-emitting efficiency 2 to 3 times as much as that of conventional fluorescence-utilizing organic light-emitting devices. However, they are still poor in saving energy or durability, and much more improvement of light-emitting efficiency and luminance have eagerly been demanded.
On the other hand, in the field of light sources for back light and illumination, there has been a strong demand for a light source which shows a high light-emitting efficiency and a high light-emitting luminance, but at present there are no better ones than fluorescent tubes. Fluorescent tubes containing mercury are causing the problem of environmental pollution and, in addition, they lack flexibility and impose limitation as to a place to provide then. Further, they have a problem of short life of as short as 10000 hours or shorter. In order to solve these problems, there have been proposed organic light-emitting devices having enough safety and enough thin thickness, which, however, are still inferior in light-emitting efficiency and light-emitting luminance. Thus, improvement in this regard has eagerly been desired.